Movement & Muscle Control

Overview
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Normal muscle movement is directed either by Reflexes (posture maintenance) or toward specific, Cortically derived goals. It always occurs in the contex of the Sensory environment, and all movements are designed with special reference to their environment.

Primary Motor Cortex

The Motor Cortex does not originate programs, its function is to integrate inputs:
• From the PreMotor Cortex
• From the two Nuclei in the Thalamus:
  • The VentroLateral Nucleus conducting Cerebellar instructions
    
  • Parts of the VentroPosterior Nucleus, conducting Proprioceptive information

Its output is directed:
• Partly to the Thalamus (VentroLateral Nucleus)
• Partly to the Cerebellum
• Mostly to the CorticoSpinal Tract

Neurons representing any one body part are arranged in column shaped nests:
• Individual muscles are represented severalfold, by Neurons that form columns at several locations
  1. Neurons that drive the most distal portions of a limb are arranged in the center
  2. Neurons driving more proximal portions are arranged in vertical Lamellae around the center

By this arrangement, columns at different sites will be surrounded by columns representing different synergists.

Those parts of the body with especially good motor abilities, ex. (Fingers and Tongue) are represented in far larger Cortical areas than parts that are capable of only less precise movements.

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These regions are located in a band adjacent and anterior to the Primary Motor Cortex. This area is:
1 - A relay station for SomatoSensory input
2 - An assembly point for Specific Motor Programs

When movement sequences are rehearsed mentally, but not performed physically, then Neurons in the Supplementary Motor Area are active and the PreMotor Cortex is quiescent.

PreMotor Cortex (Lateral Area 6):
When movement actually occurs, then the PreMotor Cortex, under instructions from the Supplementary Motor Area:
1 - Assembles specific subprograms for components of the overall movement
2 - Transmits to the Motor Areas specific motor unit activation patterns for the intended movement

Broca's Area:
This area recieves input from the language area of the Temporal Lobe. Its function is to organize Motor programs of Speech and Writing into Language patterns.

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The Primary Motor Cortex
is located in a narrow lateral band just forward of the Central Sulcus (The Precentral Gyrus). Organization is Somatotopic:
• Neurons influencing the lower body are clustered Medially
  
• Neurons influencing the head region are located Laterally
  
• Upper body Neurons are located in-between

Overview
Complex movements, requiring cooperation of many muscles, are carried out by Motor Programs that contain many subprograms. The selection of appropriate subprograms involves the Basal Ganglia as well as Command InterNeurons in area 7 of the Parietal Cortex.

The selected subprograms are then assembled and their execution is directed by Neuronal activity in the Primary Motor Cortex. A feedback path from the Motor Cortex to the Cerebellum and the Basal Ganglia ensures smooth transitions from one subprogram to the next.

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There are three Major Descending Tracts:
• CorticoSpinal
• VestibuloSpinal
• ReticuloSpinal (of Pontine or Medullary origin)

VestibuloSpinal Tract:
This Tract's fibers run on both sides of the Cord, originates in:
      1 - The Ipsilateral Lateral Ventricular Nucleus and
      2 - The ContraLateral Medial Ventricular Nucleus

These Nuclei integrate higher Motor Control patterns with Sensory information from mechano-receptors in the Vestibular organs and in muscles of the neck region.

Their output, conveyed to the Periphery via the VestibuloSpinal Tract:
Modulates activity in muscles that rotate the head and upper torso and Modulate adjustments pertinent to limb and body orientation in the gravitational field.

ReticuloSpinal Tract:
The role of ReticuloSpinal Neurons in muscle control is poorly understood. In general: They organize movements by the Shoulders, Elbows, and Face.

Two ReticuloSpinal Circuits are described:
• One relates to the role of ReticuloSpinal Neurons in the reflex by which painful cutaneous stimuli lead to limb withdrawl
• The other relates to ReticuloSpinal function in Renshaw Inhibition

When Medullary ReticuloSpinal fibers are active:
      Then Renshaw Cells are inhibited and
      Ia Inhibitory InterNeurons are disinhibited
      The resulting InterNeuron activity inhibits Alpha Motor
      Neurons of the Antagonist Muscle.

If the ReticuloSpinal Tract were cut:
      Then Renshaw Cells would be activated by Collaterals
      From, Agonist Alpha Motor Neurons.
      As a result, the InterNeurons would be Inhibited,
Leading to:
      DisInhibition of Antagonist Alpha Motor activity.
         The Antagonist would fail to relax
         during Agonist Contraction.

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CorticoSpinal (Pyramidal) Tract:
      This Tract originates in large Pyramidal Cells and
      Betz Cells (Giant Pyramidal Cells) of the Primary Cortex.
         Almost all of its nerves terminate on InterNeurons
         in the ContraLateral Gray Matter (Layers V & VI).

VestibuloSpinal Tract:
Two Tracts originate in regions of the Vestibular Nuclei:
• Superior Vestibular Nucleus projects to the OculoMotor Nuclei
  
• Lateral & Medial Ventricular Nuclei project to the Spinal Cord

ReticuloSpinal Tract:
The nerves in this Tract originate from regions in the Medial Reticular Formation of the Pons and the Medulla.
• Pontine Neurons project only Ipsilaterally
  
• Medullary Neurons project Ipsilaterally & ContraLaterally

Renshaw Inhibition:
Renshaw Cells are inhibitory to Ia Inhibitory InterNeurons. They are innervated:
• By branches of the Medullary ReticuloSpinal Tract (Inhibitory)
  
• By Collaterals from Alpha Motor Neurons (Excitatory)

Movements are accompliahed by sequences of Alpha Motor Neurons firing patterns that activate selected Motor units to contract or relax at a certain rate and to a certain tension or length. The sequences are called Motor Programs.

Motor Programs:
These sequences are learned and refined by repetitive use, allowing feedback mechanisms to make corrections whenever outcome does not match desired goals.

Motor Programs for complex movements, involving many joints, are broken down into Subprograms, and the progression from one Subprogram to the next is governed by Sensory Feedback.

Motor Programs are initiated by Command InterNeurons that act like simple on-off switches.

They are activated by the Central Nervous System after it has evaluated a constellation of Sensory and Motivational inputs.

They are located in area 7 of the Posterior Parietal Cortex. Lesions in that area are accompanied by inability to make some complex movements involving limbs ContraLateral to the lesion.

Even though other complex movements of those limbs can still be performed. Motor programs are assembled and stored in the Supplementary Motor Area.

Lesions in the Supplementary Motor Area are associated with an inability to perform movements that require a specific contribution pattern.

From several muscles in a ContraLateral limb, even though the individual muscles can be activated on command.

Following assessment of the Sensory Environment within which a given Motor Program is intended to function, four Central Nervous Areas are primarily involved in the selection and execution of Motor Programs:
• Command InterNeurons
• Supplementary Motor Area
• PreMotor Cortex
• Motor Cortex

The Events Are These:
• A Command InterNeuron conveys instructions that allow the Supplementary Motor Area to select the correct Motor Program

• The Basal Ganglia supply to the Supplementary Area information that:
  • Allows selection of programs for appropriate preparatory movements
  • Ensures efficient sequencing & deployment of muscle activity

• The PreMotor Cortex
  • receives assembled global instructions from the Supplementary Motor Area
  • fills in the details & activates specific Motor Cortex Neuron

• The Motor Cortex
  • uses action information from the PreMotor Cortex and
  • uses timing information from the Cerebellum to activate specific motor units

Execution of every Motor Program is preceded by an assessment of the Sensory Environment. This occurs in the SomatoSensory Cortex and the Visual Cortex.

On This Basis Of The Environment:
1 - A Command InterNeuron instructs the Supplementary Motor Area, which Motor Program is appropriate.
2 - The Supplementary Motor Area also receives extensive input from the Basal Ganglia (via VentroLateral, VentroAnterior,and MedioDorsal Thalamic Nuclei).

Its Purposes Are:
• Selection of preparatory movement programs and
• Planning the most efficient deployment of muscle activity

Thus, the Supplementary Motor Area assembles global instructions for movements that involve participation of many muscles in a specific pattern and sequence.

It issues these instructions to the PreMotor Area.

The PreMotor Cortex fleshes out the details of smaller components of the global instructions received from the Supplementary Motor Area.

And then activates specific Motor Cortex Neurons. Motor Cortex Neurons activate specific motor units.

The timing of the activation-deactivation sequences is determined by Cerebellar activity, reaching the Motor Cortex via VentroLateral Thalamic Nuclei.

The consequences of motor unit activation are detected by MechanoReceptors in the affected muscles.

They are conveyed to the SomatoSensory Cortex and from there:
• to the Cerebellum, via Mossy and Climbing Fibers;
• to the Basal Ganglia,
• to the SubThalamic Nucleus and Substantia Nigra Compacta;
• to the Command InterNeuron area of the Cerebral Cortex

This Reafferentation gauges the relative success or failure of the Motor Act and initiates appropriate modifications in subsequent motor unit activity.